Enthalpy of a reaction

Because these various quantities are characteristics of the reactants and products but are independent of the reaction path, they cannot provide insight into mechanisms. Information about AG, AH, and AS does, however, indicate the feasibility of any specific reaction. The enthalpy change of a given reaction can be estimated from tabulated thermochemical data or from bond-energy data such as those in Table 1.3 (p. 14) The exan le below illustrates the use of bond-energy data for estimating the enthalpy of a reaction. 

EXAMPLE 6.14 Using mean bond enthalpies to estimate the enthalpy of a reaction... 

Use standard enthalpies of formation to calculate the standard enthalpy of a reaction, and vice versa (Examples 6.11 and 6.12). 

Expressed in this form, the Gibbs-Helmholtz equation is widely used in experimental thermodynamics to determine AH, the enthalpy of a reaction, from the experimentally de-... 

Enthalpies are often used to describe the energetics of bond formations. For example, when an amide forms through the condensation reaction between an ester and an amine, the new C-N bond, has an enthalpy of formation of -293 kj/mole. The higher the negative value for the bond enthalpy of formation, the stronger the bond. An even more useful concept is the enthalpy of a reaction. For any reaction, we can use the fact that enthalpy is a state function. A state function is one whose value is independent of the path traveled. So, no matter how we approach a chemical reaction, the enthalpy of the reaction is always the same. The enthalpy of... 

According to this theory, the activation energy of a reaction is high when the enthalpy of a reaction is high, as demonstrated in Fig. 1. 

The enthalpy that accompanies a change of physical state at standard conditions is called the standard enthalpy of transition and is denoted A trsH°. Enthalpy changes accompanying chemical reactions at standard conditions are in general termed standard enthalpies of reaction and denoted A r H0. Two simple examples are given in Table 1.3. In general, from the first law, the standard enthalpy of a reaction is given by... 

Although periodic trends in enthalpies of formation are often striking, these trends can in general not be used to estimate accurate data for compounds where experimental data are not available. Other schemes are frequently used and these estimates are often based on atomic size and electronegativity-related arguments. As an example, the enthalpy of formation of a ternary oxide from the binary constituent oxides, i.e. the enthalpy of a reaction like... 

Adiabatic calorimetry uses the temperature change as the measurand at nearly adiabatic conditions. When a reaction occurs in the sample chamber, or energy is supplied electrically to the sample (i.e. in heat capacity calorimetry), the temperature rise of the sample chamber is balanced by an identical temperature rise of the adiabatic shield. The heat capacity or enthalpy of a reaction can be determined directly without calibration, but corrections for heat exchange between the calorimeter and the surroundings must be applied. For a large number of isoperibol... 

In section 5.1, you learned about the energy changes that accompany physical changes, chemical reactions, and nuclear reactions. You learned how to represent energy changes using thermochemical equations and diagrams. In the next section, you will determine the enthalpy of a reaction by experiment. 

This value of AH° is the same as the value you obtained using AH° data. When you used the addition method, you performed the same operations on the enthalpies of formation before adding them. Therefore, using enthalpies of formation to determine the enthalpy of a reaction is consistent with Hess s law. Figure 5.17 shows the general process for determining the enthalpy of a reaction from enthalpies of formation. 

The primary signihcance of bond enthalpies lies in the calculation of the enthalpy of a reaction involving a compound for which no enthalpy data are available. For example, if the enthalpy of formation of Se2Cl2(g) were not known, it could be calculated from bond enthalpies by the following steps. As the bond enthalpy refers to the dissociation of Cl-Se-Se-Cl gas into gaseous atoms, the enthalpy change for the formation of this gaseous molecule from the atoms should... 

In the preceding sections we discussed methods of obtaining enthalpies of reaction at a fixed temperature (generally 298.15 K). In particular, we pointed out that it is possible to tabulate enthalpies of formation and bond enthalpies and to use these to calculate enthalpies of reaction. Such tables of enthalpies of formation are available for only a few standard temperatures. Frequently, however, it is necessary to know the enthalpy of a reaction at a temperature different from those available in a reference table. Therefore, we consider now the procedures that can be used to calculate the enthalpy of reaction (at constant pressure) at one temperature, from data at another temperature. 

The enthalpy of a reaction can be obtained experimentally with the aid of the varit Hoff equation by measuring the equilibrium constant, K, over a range of temperatures and plotting InK against 1 /T to give a straight line the slope of which is AH /R. 

A Born-Haber cycle is the application of Hess s Law to the enthalpy of formation of an ionic solid at 298 K. Hess s law states that the enthalpy of a reaction is the same whether the reaction takes place in one step or in several. A Born-Haber cycle for a metal chloride (MCI) is depicted in Figure 1.56 the metal chloride is formed from the constituent elements in their standard state in the equation at the bottom, and by the clockwise series of steps above. From Hess s law, the sum of the enthalpy changes for each step around the cycle can be equated with the standard enthalpy of formation, and we get that ... 

What is the meaning of enthalpy of a reaction Explain with a formula. 

Hess s law The overall enthalpy of a reaction is the sum of the reaction enthalpies of the steps into which that reaction can be divided. 

Enthalpy of a reaction depends upon the following factors ... 

In this second type of sample, the enthalpy of a reaction can be determined from the heat of the reaction, which will take place at constant atmospheric pressure. 

Born-Haber s cycle — Hess s law establishes that the enthalpy of a reaction is the same independently whether the reaction proceeds in one or several steps. It is a consequence of the first law of thermodynamics, which states the conservation of energy. Born and -> Haber applied Hess s law to determine the - enthalpy of formation of an ionic solid. The formation of an ionic crystal from its elements according to Born-Haber s cycle can be represented by the following diagram. 

Many reactions are difficult to calculate with the standard enthalpies of formation or calorimetry. Enthalpy is a state function that means that the calculation is independent of the pathway or calculation. Hess s Law states that an enthalpy of a reaction can be calculated from the sum of two or more reactions. 

Hess s Law An enthalpy of a reaction can be calculated from the sum of two or more reactions. 

The enthalpy of a reaction can be calculated in two ways (1) from the enthalpies of formation of products and reactants and (2) by combining enthalpies of any reactions (using Hess s law). 

If the apparent equilibrium constant and standard transformed enthalpy of a reaction are measured at a temperature different from 298.15 K and the species properties are known for all the reactants but one at 298.15 K, the question is how to calculate the species properties of that one reactant at 298.15 K for entry into a database. First we consider the simplest case where the reactant with unknown species properties consists of a single species. The reaction chosen as an example is EC 3.5.I.3. 

Write the formula that can be used to calculate the enthalpy of a reaction from the enthalpies of formation of the reactants and products. (16.4)... 

Thus, the (standard) enthalpy of a reaction is obtained by taking the difference between the (standard) enthalpy of formation of products and reactants. If the (standard) enthalpy of reaction or formation is negative (exothermic), as is the case with most combustion reactions, then energy is liberated due to the chemical reaction. Energy is absorbed if A// is positive (endothermic). 

The enthalpy of a reaction is defined as heat released or adsorbed when several reactants (compounds or elements) react to form products. If the reaction involving components 1 through j occurs, the general form may be expressed as... 

In the above example, the reaction enthalpy is calculated at 25°C (77°F) and 1 atm. It is usually called the standard enthalpy of reaction. In the most cases, the reaction is not at standard temperature and pressure (STP 25°C and 1 atm). The reaction enthalpy of a reaction at any temperature and pressure for a reaction is still calculated by the enthalpy difference before and after reaction,... 

The enthalpy of a reaction, 6B°, is the heat transfer between a system and its surroundings for a process at constant pressure, but not at constant temperature and volume (VO. For example, consider the formation of liquid water from gaseous hydrogen and oxygen at 25°C, which, with respective volumes and AHf values given beneath it is written... 

Write an expression for the enthalpy of a reaction in terms of the enthalpies of products and reactants. Under what condition is the heat of a reaction equal to the enthalpy change of the same reaction ... 

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